Non-photoemissive grid for a phototube and process for making same



June 1967 A. i... GREILICH ,32

NON-PHOTOEMISSIVE GRID FOR A PHOTOTUBE AND PROCESS FOR MAKING SAME Filed Dec. 25, 1964 7145 PULSE GENERATOR INVENTOR. ALFRED L. GPE/L/CH ATTORNEY United States Patent 3,327,152 NON-PHOTOEMISSIVE GRID FOR A PHOTOTUBE AND PROCESS FOR MAKING SAME Alfred L. Greilich, Livermore, Califl, assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Dec. 23, 1964, Ser. No. 420,837 7 Claims. (Cl. 313-99) The present invention relates to a process for treating electron tube elements and to tubes employing the treated elements therein. More specifically, this invention relates to a process for rendering a surface of tube elements non-photoemissive and to a grid-controlled phototube having a non-photoemissive control grid.

In the past, the presence of dark currents have frustrated attempts to make grid-controlled phototubes whose anode currents could be effectively cut off by biasing the control grid negative with respect to a photoemissive cathode. This dark current, as the term is used herein, is that current which ordinarily flows to the anode in the presence of a bias between the control grid and photoemissive cathode of a magnitude which normally would be expected to inhibit photo-induced current flow from the cathode to the anode. It has been found that the dark current originates from the control grid and such current exists because those processes commonly used to activate the photoemissive cathode of a phototube are of such a nature that the control grid as well as the photocathode is rendered photoemissive. In a typical process for activating a photoemissive cathode, a film of appropriate photoemissive material, such as antimony, is deposited on a metal substrate which provides structural support for the photoemissive cathode. This substrate along with an anode and a grid is mounted in a glass envelope prior to activation of the cathode to form the phototube. The glass envelope is then evacuated and an evaporated photoactivating material, such as cesium, is introduced into the tube. This cesium or other similar material migrates to the substrate and reacts with the antimony evaporated thereon thereby forming a photoemissive cathode having a low work function surface. During this process the cesium must diffuse through the mesh openings in the grid in order to reach the cathode surface. Consequently, a considerable amount of cesium is deposited on the grid thereby rendering that grid also photoemissive. Although this cesiated grid is not nearly so photosensitive as the cathode, nevertheless, its work function is reduced to a point Where the above-mentioned dark currents originating therefrom substantially degrade tube performance. Although an electrostatic field established by a negative grid-cathode bias voltage may be sufiicient to repel substantially all photoelectrons emitted from the cathode, since the grid is photoemissive, it is capable of producing photoelectrons which create an anode current.

The present invention provides a unique process of treating the grid electrode of a grid-controlled phototube, or any other material structure, to render the structure non-photoemissive. The term photoemissive as used herein will refer to photosensitivities equal to or greater than six microamperes per lumen per square inch of emission surface. The term non-photoemissive will relate to photosensitivities which are substantially less than the above noted six microampere level. In the process of the invention, the surface of the structure to be rendered nonphotoemissive is initially coated with a material which when exposed to a selected photoactivating agent will interact therewith to form on the surface a coating having a work function whose magnitude is greater than or which is not reduced more than one order of magnitude 3,327,152 Patented June 20, 1967 less than that of the unprocessed structure material. The maintenance of a high work function on the surface of the material renders the surface substantially non-photoemissive.

A material which will so interact will be termed hereinafter as a photoemissive retardant." After applying the photoemissive retardant to the surface, the photoactivating agent is coated thereover whereby a reaction is believed to occur between the two coatings to create thereby a non-photoemissive surface.

Materials which will serve as appropriate photoemissive retardants depend upon the particular photoemissive activating agent being used. Photoemissive activating agents can be selected from numerous materials, such as, the alkali metals of cesium, sodium, potassium, rubidium, etc. Where cesium is employed as the activating agent, materials such as iron, tin, lead and chlorides of nickel are used as the photoemissive retardants.

By treating the grid electrode of a grid-controlled phototube in accordance with the process of the present invention, a grid-controlled phototube characterized by the substantial absence of the undesirable dark current is produced.

Accordingly, the primary object of the present invention is to provide a process in which a surface inadvertently exposed to a photoemissive activator is retained in a non photoemissive state.

It is a more particular object of the present invention to provide a process for rendering a surface photoemissive while objects proximate the surface are retained in a non-photoemissive state.

It is a further object of the present invention to provide a grid-controlled phototube having a greatly decreased level of dark current.

It is a more particular object of the present invention to provide a grid-controlled phototube having substantially no dark currents.

It is yet another object of the present invention to provide a grid-controlled phototriode tube whose grid electrode is non-photoemissive.

These and other objects of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 isometrically depicts a grid-controlled phototriode tube provided with a non-photoemissive grid in accordance with the invention; and

FIGURE 2 schematically depicts a phototriode tube and typical associated circuitry.

Referring to FIGURE 1, phototriode tube 11 typically includes a photoemissive cathode electrode 12 mounted within a hermetically sealed envelope 13. To provide a light path through envelope 13 to the photoemissive surface 14 of cathode electrode 12, envelope 13 includes a central portion 16 composed of light pervious material such as glass arranged in facing relationship with electrode 12. An anode electrode 17 is rigidly secured in spaced, insulated relation to cathode electrode 12 substantially axially within envelope 13 to receive photoelectrons liberated from photoemissive surface 14 as a result of light impinging thereon. Towards effectively controlling the current flow from cathode electrode 12 to anode electrode 17, a mesh screen grid electrode 18, pervious to electron passage, is disposed coextensively between and insulated from anode electrode 17 and cathode electrode 12. Grid electrode 18 is positioned proximate the photoemissive surface 14 of cathode electrode 12 to, in effect, electro-statically shield cathode electrode 12 from anode electrode 17. The tubes anode cathode and grid electrodes, 17, 12 and 18 respectively, are provided with conductive extensions 19, 21, and 22 which transpierce envelope 13 in hermetically sealed insulated relation thereto for connection to exterior circuitry.

However, because of the particular manner in which the.

cathode activation process is conventionally carried out (as fully described in the literature, for example, Zworykin and Ramberg, Photoelectricity, published by John Wiley and Sons, Inc., 1949), the treating process results in rendering the grid electrode 18 to be photoemissive; In such a conventional cathode activating process, the tube 11 is baked out at 275 C. until a pressure of about =1 10 mm. Hg is attained. The tube 11 is then brought to about a 160 C. processing temperature and allowed to stabilize thereat. Cesium vapor, or other appropriate photoemissive activating material, is generated in a side-arm (not shown) and introduced into tube 11 through pump out tabulation 23 by differential heating. This cesium diffuses through grid electrode 18 to impinge on cathode electrode 12 and combine with antimony deposited on the surface of cathode electrode 13 which is to be exposed to light. The cesium and antimony react to form the photoemissive cathode surface 14 of cesium antimonide, this photoemissivity being the concomitant of the extremely low work function of the cesium-antimony combination. In the process of activating cathode electrode 12, some of-the diffusing activating material, i.e., cesium in this case, settles on grid 18 instead of passing through the grid to deposit on the surface of cathode electrode 12. As noted previously, in the past such deposition of the cesium on the grid structure 18 rendered the grid photoemissive.

However, in the process of the present invention, the grid 18 is treated to provide a coating of certain select retardant agents thereon prior to activation whereby the subsequently deposited cesium forms a non-photoemissive surface on grid 18. With alkali metal activators, effective retardant agents include iron, tin, lead and chlorides of nickel (-NiCI The preferred procedure for applying such materials is by means of a vacuum vapor deposition. However, iron has been found to be most effective as a retardant grid coating when cesium is employed to activate the cathode electrode of a grid-controlled phototube so as to substantially eliminate undesired photoemissivities of the grid 18. It is thought that the iron interacts with the subsequently deposited cesium in some unexplained manner to raise the work function of the surface to eliminate the photoernissivity. While the metallic elements are generally applied by a vapor plating or vacuum vaporization process, it has been found that the nickel chloride may also be applied if a nickel grid isemployed merely by dipping the grid into diluted hydrochloric acid. Evidently sufiicient chloride is adsorbed or otherwise is retained by the nickel surface to be effective.

In the preferred embodiment described herein, the nonphotoemissive zone of interaction product is formed by depositing cesium on at least a monomolecular layer of pure iron. However, a similar eifect will be obtained by arranging for other photo-activating agents to deposit on a layer of appropriate photoemissive retardant and thereby render the surface non-photoemissive.

In the preferred process of rendering the grid electrode 18 of the grid-controlled phototube 11 non-photoemissive, oxide-free iron is vacuum deposited by well-known conventional techniques onto grid 18 to cover the surface thereof with at least a monomolecular thickness layer of iron. The electrodes, i.e., anode 1-7, cathode 12 and iron coated grid 18, are thenrassembled within envelope 13,,

and the envelope 13 hermetically sealed and evacuated as in conventional practice. To prevent outgassing prob-' lems which could poison the photoemissive cathodeduring operation, iron coated grid 18 should be exposed to the atmosphere as little as possible during the fabrication of phototube 11. To assure minimum exposure of the iron coated grid 18 to the atmosphere, both the coating process and assembly of phototube 11 can be completed within a single vacuum chamber thereby never exposing the iron coated grid electrode 18 to the atmosphere. The

coating of iron can conceivably be applied to the grid surface, e.g., of nickel by any method which is suitable for obtaining an oxide free deposit. Possibly a grid formed of iron could be treated with reducing agents or by heating in a vacuum to yield a grid inherently capable of retarding photoemissivity. In any event, vapor deposition performed under high vacuum conditions in which a heated wire or pool of molten iron is used to produce the vapor,

has been found eminently suitable. The other agents mentioned herein are deposited similarly. The surface of cathode electrode 12 is then activated to form the photoemissive cathode surface 14 in the manner noted supra.

The process of the present invention may be employed to fabricate any of the various grid-controlled phototubes, e.g., a grid-controlled electron multiplier tube or a gridcontrolled. photoelectronic image tube. However, the process is most important in the construction of a fast, highcurrent phototriode tube which is capable of being gated on in less than 3 nanoseconds, with the on-off current ratio being on the order of 103.

With particular reference vto FIGURE 1, using the typical elements described above, the cathode 12 of a fast highcurrent phototriode tube is comprised of a semicylindrical tube 24. of conducting material, such as Kovar (a nickelcobalt-iron alloy manufactured by Westinghouse Electric Corporation) having first and second cylindrical ends 26 and 27. A semicylindrical sheet substrate 28 comprised of a metal such as nickel, chromel or alumel, is rigidly secured, e.g., by spotwelding, in nested relation to the inner surface of tube 24. This substrate supports the photoemissive cathode surface 14 next to be described. In preferred practice antimony is evaporated to deposit on the inner surface of substrate 28 to a film thickness corresponding to approximately 58% light transmission. The cathode electrode 12 is completed by activating its surface with cesium thereby forming the photoemissive surface 14 of cathode 1 2 as described above.

Cathode electrode 12 is rigidly mounted within cylindrical glass envelope 16 having first and second supporting metal tapered tubular members 29 and 31 sealed respectively at each end and engaging the cylindrical ends 26, 27 of tube 24. Cathode electrode 12 is mounted within envelope 13 by hermetically inert gas arc-welding ends 26 and 27 to members 29 and 31 respectively.

Anode electrode 17 is provided with a blade-shaped configuration and is rigidly mounted within envelope 13 with a narrow edge 32 facing and spaced from the photoemissive surface 14 of cathode electrode 12. Anode electrode 17 is secured in position by'a conducting mounting post 33 extension thereof which penetrates an insulating seal support 34 which is rigidly mounted to the inner surface of cylindrical end 26 of tube 24.

Grid electrode 18 is fabricated from a nickel mesh screen of 84% open transmission ratio. The screen is formed into a semicylindrical configuration and is fused to a conforming nickel frame 37. At least, the inner sur face 39 of semicylindrical screen grid 18 is covered by a first layer of oxide-free iron in accordance with the invention. The layer of iron is in turn covered by a layer of cesium which interacts therewith to form a non-photo emissive control grid 18. Control grid 18 is disposed between anode electrode 17 and cathode electrode 12 proximate to and in spaced nested relation with photoemissive cathode surface 14. For most efficient operation, the entire photoemissive cathode surface 14 is shadowed by screen grid 18 to thereby electrostatically shield cathode of tube 24. Insulator seals 34 and 42 further serve to hermetically seal envelope 13.

To provide the means of applying selective voltages to the tubes electrodes, each electrode, i.e., anode 17, cathode 12 and grid 18 are provided with conductive extensions 19, 21 and 22 respectively which hermetically transpierce to the exterior of envelope 13. Anode extension 19 is a conductive extension of post 33 which c0- axially hermetically transpierces seal 34. Extension 22 of grid 18 is that portion of post 41 which coaxially hermetically transpierces seal 42. Cathode electrode 12 is provided with two conductive extensions, 21 and 21', which hermetically extend respectively through opposite ends of envelope 13 to the exterior thereof. Conductive extension 21 is formed by member 29 and the cylindrical extension 26 of cathode electrode 12. Conductive extension 21 coaxially surrounds anode extension 19 to form a coaxial connector. Similarly, conductive extension 21' is formed by member 31 the cylindrical extension 27 of cathode electrode 12. The conductive extension 21' coaxially surrounds grid extension 22 to form another coaxial connector. Hence, a photot-riode tube is provided with coaxial input terminals to the grid-cathode portion of tube 11 and coaxial output terminals to the anode-cathode portion of tube 11.

FIGURE 2 shows tube 11 as connected in a conventional electronic circuit. In the figure, cathode 12 connects to the negative terminal of power source 43, to the positive terminal of power source 44, and to the positive terminal of pulse generator 46. The positive terminal of power source 43 connects through re'isto-r 47 to anode 17. The negative terminal of power source 44 connects through resistor 48 to control grid 18 and the negative terminal of pulse generator 46 also connects to grid 18 through resister 49.

In operation, light, depicted by arrow 51, penetrates through glass envelope 13 and impinges on grid 18 and cathode 12. The energy of the light is transferred to the electrons of such surfaces. Since the surface 14 of the cathode 12, as described above, has a low work function, those electrons whose energy is sufficiently increased by the light bombardment are able to overcome the surface energy potential barrier and escape into the evacuated region of the phototube. The negative potential imposed on grid 18 by power supply 44 repels these electrons and hence cuts olf current to anode 17. When grid 18 is pulsed positive with respect to cathode 12 the photoemitted electrons are able to pass the grid and are accelerated to positively biased anode 17. These electrons flowing to anode 17 set up a current which passes through resistor 47. The number of electrons flowing are proportional to the intensity of the incident light 51. Hence, by an Ohms law calculation, it is seen that the voltage appearing across resistor 47 is proportional to the intensity of the incident light.

With this circuit arrangement it is possible to observe an illuminated event over the time duration defined by the pulse of generator 46. As mentioned earlier. grid 18 was pretreated to provide a layer of vapor deposited iron such that the cesium deposited thereon interact with this iron to provide a high Work function surface. With this high work function surface, the light impinging on the concave surface of grid 18 does not impart sufficient energy to the electrons within the grid to allow the electrons to overcome the surface barrier and escape into the evacuated region of the tube. Hence, there are none of the before mentioned dark currents in this tube.

An on-otf current ratio is a convenient figure of merit to illustrate the effectiveness of the present invention. In the preferred embodiment described immediately above, use of the iron coated control grid 18 resulted in an on-off current ratio in excess of 140021. This is in sharp contrast with current ratios of less than 200:1 found in phototriodes not utilizing the novel iron coated grid 18. A comparative analysis of these two on-otf ratios reveals a greater than seven to one reduction in dark current attributable to the use of the pretreated grid.

The use of a blade-shaped anode 17 having an elongated cross section serves to improve the response of the present phototube to those pulses imposed on control grid 18. There is a tendency for a substantial portion of the photoelectrons to miss the front narrow edge 32 of anode 17. Now, should this anode be comprised of a slender member, these electrons would spiral around the anode whereby they would gradually approach it. The duration of this spiralling represents a degradation in the response of the tube to those on pulses impressed on grid 24. By simply incorporating an elongated cross section feature to the anode, this spiralling effect is eliminated. Although the electrons may initially miss the front edge 32 of the anode 17, they immediately are intercepted by the extended portion of the anode 17, thus eliminating the spiralling transit time.

It may be worth noting that this elongated cross sec tion anode feature and the coaxial feature of this phototriod are described in substantial detail in the inventors copending application Ser. No. 237,777 filed, Nov. 14, 1962, entitled, Ruggedized Photodiode Tube.

At this point it should be emphasized that the concept of pretreating a surface with a material that will react with a photoemissive material introduced into the tube so as to produce a non-photoemissive high work function sur face is not limited in its application to the grid. The concept is applicable in any setting where the formation of photoemissive surfaces is desired to be prevented.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention 'm its broader aspects and therefore the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

\Vhat is claimed is:

1. In a process for forming a photoemissive cathode of a grid-controlled phototriode tube without rendering the grid photoemissive, the steps comprising,

(a) directing a photoemissive retardant vapor onto a surface of a grid electrode defining mesh openings to form at least a monornolecular coating of photoemissive retard-ant on said surface,

(b) positioning said grid electrode proximate a cathode electrode of said phototriode tube with said coated surface of said grid electrode distal said cathode electrode, and

(c) directing a vaporized photoactivating material towards said cathode from a region on the side of said grid electrode distal said cathode whereby the coated surface of said grid electrode intercepts a portion of said vaporized photoactivating material directed towards said cathode, said photoemissive retardant deposited on said grid interacting with said intercepted photoactivating material to form a non-photoemissive zone of reaction products, said photoactivating material vapor passing through said mesh openings of said grid settling on the surface of said cathode proximate said grid to render said surface photoemissive.

2. In the process recited in claim 1 further defined by said grid and cathode electrodes being coated in an oxygen free environment, said photoemissive retardant vapor being iron, and said photoactivating vapor being cesium.

3. In a phototube including at least one photoemissive cathode electrode and at least one non-electronemissive anode electrode positioned to receive photoelectrons emitted from the photoemissive surf-ace of said cathode in response to light impinging thereon, the combination therewith comprising,

(a) a mesh grid disposed proximate the electron emitting surface of said cathode electrostatically between said cathode and anode electrodes, the surface of said grid facing said light that is to impinge upon said cathode coated With a non-photoemissive interaction product of a photoemissive retardant and a photoemissive activating agent.

4. A grid-controlled phototriode tube comprising,

(a) a hermetically sealed envelope,

(b) a photoemissively activatedcathode electrode disposed Within said envelope and having a surface responsive to light impinging thereon by the emission of electrons,

(-c) a non-elec-tronemissive anode electrode positioned Within said envelope insulated apart from said cathode to receive said electrons emitted by said cathode,

(d) a mesh grid electrode disposed proximate the surface responsive to light, the surface of said grid distal said cathode coated with a non-photoemissive interaction product of a photoemissive retardant and a photoemissive activating agent, and

(e) connector means connected to said anode, cathode and grid electrodes and exteriorly extending through said envelope in hermetic sealed relation thereto.

5. The phototriode tube recited in claim 4 further defined by said non-photoemissive reaction product formed by the reaction between a layer of cesium deposited on a layer of oxide-free iron.

6. A grid-controlled phototriode tube comprising,

(a) a hermetically sealed light-pervious envelope,

(b) a semicylindrical cathode disposed Within said envelope and having its concave surface responsive to light impinging thereon by the emission of electrons,

(c) a blade-shaped anode positioned Within said envelope with a narrow edge facing and having a common plane of symmetry With said semicylindrical cathode,

(d) a semicylindrical mesh grid electrode of conductive material disposed proximate to and in nested relation with said concave surface of said cathode, the surface of said grid electrode distal said cathode coated With a non-photoemissive interaction product formed by the reaction between a layer of cesium deposited on a layer of oxide-free iron,

(e) a first coaxial connector having inner and outer conductors, said inner and outer conductors connected to proximate ends of said grid and cathode electrodes respectively and hermetically transpiercing to the exterior of afirst end of said envelope, and

(f) a second coaxial connector having inner and outer conductors, said inner and outer conductors connected to proximate ends of said anode and cathode electrodes respectively and hermetically transpiercing to the exterior said envelope through the end opposite said first end of said envelope.

7. The process recited in claim 1 further defined by said photoemissive retardant vapor selected from the vapors consisting of iron, lead and chloride of nickel vapors.

References Cited UNITED STATES PATENTS 2,103,498 12/1937 Schroter 313-97 X 2,556,864 6/1951 Apker 313107 X 2,905,843 9/ 1959 Lubszynski 313-355 X 3,070,721 12/1962 Ferry et a1. 313355 X JAMES W, LAWRENCE, Primary Examiner.

R. J UDD, Assistant Examiner. 

3. IN A PHOTOTUBE INCLUDING AT LEAST ONE PHOTOEMISSIVE CATHODE ELECTRODE AND AT LEAST ONE NON-ELECTRON-EMISSIVE ANODE ELECTRODE POSITIONED TO RECEIVE PHOTOELECTRONS EMITTED FROM THE PHOTOEMISSIVE SURFACE OF SAID CATHODE IN RESPONSE THE LIGHT IMPINGING THEREON, THE COMBINATION THEREWITH COMPRISING, (A) A MESH GRID DISPOSED PROXIMATE THE ELECTRON EMITTING SURFACE OF SAID CATHODE ELECTROSTATICALLY BETWEEN SAID CATHODE AND ANODE ELECTRODES, THE SURFACE OF SAID GRID FACING SAID LIGHT THAT IS TO IMPINGE UPON SAID CATHODE WITH A NON-PHOTOEMISSIVE INTERACTION PRODUCT OF A PHOTOEMISSIVE RETARDANT AND A PHOTOEMISSIVE ACTIVATING AGENT. 